1. Field of the Invention
This invention relates generally to a predistortion linearizer circuit useful for altering a signal prior to its input to a power amplifier in order to better linearize the output of the amplifier, and, more particularly, to a predistortion linearizer circuit with electronic tuning to linearize the power amplifier output.
2. Description of the Related Art
High power amplifiers are routinely employed in many applications to amplify a signal to a power level sufficient for its intended transmission. For example, high power amplifiers are commonly employed in base station applications to amplify a signal for transmission. Solid state power amplifiers and traveling wave tube amplifiers are just two examples of high power amplifiers that have been used in these applications.
Although these high power amplifiers impart sufficient gain to the amplified signal such that the signal may be transmitted along its intended path, the gain through the amplifier is typically not constant, but rather varies as a function of the input signal level. As a general rule, as the input signal power increases, the amplifier will introduce a gain expansion followed by gain compression. In addition, the input signal experiences a phase shift through the high power amplifier, and the amount of phase shift is also a function of the input signal level. Generally, the phase will initially lead then lag with increasing input power. These amplitude and phase nonlinearities introduced by the high power amplifier distort the transmitted signal and, accordingly, the information carried by the signal.
Predistortion linearizer circuits have been employed to offset the distortion introduced by the power amplifier. A predistortion linearizer circuit introduces a distortion to the signal to be transmitted prior to amplification of the signal in the amplifier. The distortion introduced by the predistortion linearizer circuit ideally is the inverse of the distortion the amplifier will introduce, at least over some specified range of input signal power and range of operation of the amplifier. That is, a gain and phase nonlinearity introduced by the predistortion linearizer circuit will cancel the gain and phase nonlinearity introduced by the amplifier over the specified range, and the output of the amplifier will be relatively linear over that range.
Various predistortion circuits have been described in the prior art. However, those circuits each suffer from one or more undesirable characteristics. For example, certain predistortion linearizer circuits are too large because of their complexity. Other predistortion linearizers, while not as large, do not provide a sufficiently complex transfer characteristic to yield the linearized output at the high power amplifier that is desired. In general, predistortion linearizers have not been flexible in their gain and phase response, being incapable of adjusting to changing conditions of operation to maintain optimum performance characteristics.
The present invention is directed to overcoming, or at least reducing the effect of, one or more of the problems set forth above.